This page, with accompanying slideshows and photos, is the most comprehensive varnishing video I've ever seen. It's put together by NitpickyArtist.com, which has a lot of good technical info including advice on how to reduce glare, how to enter juried shows, or even info about table surfaces and easels. You may not want to follow this person's advice all the way through, as it's quite complex, but there are a ton of good ideas in this article to use otherwise. It was the section at the bottom on how to clean a varnish brush that caught my interest initially.